Copolymers of non-modified 2,2'-(1,3-phenylene)bis (oxazoline) and phenolic novolak resins in which the bisoxazoline monomer acts as a cross-linking agent (and secondarily as a chain extending agent) have been reported in the literature. These copolymers offer high values of modulus, strength, toughness, heat deflection temperature and interlaminar shear (i.e. strong bonding to carbon fibers and fiberglass). The reported bisoxazoline and phenolic novolak copolymers also have much better thermo-oxidative stability than epoxy resins and almost all other thermoset resins, with the exception of polyimides.
Polyimides have higher thermo-oxidative stability than reported bisoxazoline and phenolic novolak copolymers; however, polyimides do not have the necessary strength for high performance airframe (non-engine) applications. A polyimide probably would have the thermooxidative stability required for supersonic commercial or military aircraft parts exposed to surface skin temperatures of about 177.degree. C. (350.degree. F.) and above. However, because of microvoids, microcracking, etc., a thermoset polyimide would be not be strong enough for adequate service life of large supersonic jet parts, such as wings and tail sections.
The material used to manufacture such parts must have thermo-oxidative stability at temperatures above about 177.degree. C. (350.degree. F.) because new products require such features. For example, the new supersonic NASA HSCT (high speed civil transport) will operate for many hours (per flight) at speeds of about Mach 2.2 with skin temperatures of at least about 177.degree. C. (350.degree. F.). Supersonic military fighter aircraft also may operate with at least about 177.degree. C. (350.degree. F.) skin temperatures, but for shorter times than in the HSCT case. Other applications requiring thermo-oxidative stability at temperatures above about 177.degree. C. (350.degree. F.) include: high temperature electrical applications, such as transformer housings, submersible vessel support structures (i.e., near nuclear reactors), certain steel manufacturing applications (i.e., for high temperature gaskets), etc.
Unfortunately, known thermoset resins that have the necessary strength for such parts do not also have the increased, long term, thermal oxidative stability required for components used in high performance, high temperature applications. Bisoxazoline and phenolic novolak copolymers possess the requisite strength and have much better thermo-oxidative stability than many resins; however, known bisoxazoline and phenolic novolak copolymers do not have sufficient thermo-oxidative stability for many of these high performance, high strength applications. Consequently, a need exists for a thermoset resin which has increased, long term, thermal oxidative stability, as well as other desirable characteristics, such as high values of modulus, strength, toughness, heat deflection temperature, and interlaminar strength.